Spectral phase is vital to retrieve both the real and imaginary parts of both second-order vibrational and electronic sum frequency generation spectra. Despite recent efforts, phase measurements in electronic spectra at interfaces are left behind. In particular, references are not well established for frequency-dependent spectral phases in interfacial electronic spectroscopy. In this work, we present broadband heterodyne detection of interfacial electronic spectroscopy (HD-iES) for spectral phases of references. Such a broadband HD method was based upon a broadband short-wave IR laser source from 1100 to 2200 nm. A collinear geometry was adopted for passivated phase stability. We further compared often-used reference samples, including gold (Au) thin films, silver (Ag) thin films, <i>p</i>-type GaP (100),  <i>p</i>-type GaAs (100), as well as left-handed and right-handed z-cut a-quartz crystals. The phases of the second order susceptibilities for the Au thin films are frequency dependent, while the phases of the second order susceptibilities for Ag thin films, the GaP crystal, the GaAs crystal, and the quartz crystals are independent of frequency of our interest. However, orientations of the GaP crystal, the GaAs crystal, and the quartz crystals require dedicated calibrations since their spectral phases of second order susceptibilities depend on azimuthal angles. Of them, Ag thin films are the best choice for the phase reference in interfacial second-order spectroscopy.

Phase measurements from broadband heterodyne detection in interfacial electronic spectroscopy

ACS Spring 2022

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